Present-day commercial and military aircraft incorporate highly complex electronic control systems incorporating numerous sensors, force transducers and servo systems, as well as the electronics necessary for processing the sensor signals and developing the requisite control signals for the transducers and the like so that the aircraft can be flown in a controlled manner. Typically, the electronic assemblies involved will be housed in metallic shielding enclosures or boxes which are adapted to slide into equipment racks on the aircrafts.
It is well known that electromagnetic radiation may interfere with the operation of electronic assemblies. Accordingly, it is important to provide an electromagnetic interference (EMI) shielding enclosure around each of the electronic assemblies. Ideally, the enclosure for each peripheral device is designed as a Faraday cage. A Faraday cage typically includes a sixsided box made of metal or other conductive material. The interfacing of the sides to each other is referred to in the industry as xe2x80x9cEMI seams.xe2x80x9d To be effective, a Faraday cage should have xe2x80x9ctightxe2x80x9d EMI seams, that is, the interface of two sides of the cage should reduce or eliminate EMI so that it is not able to penetrate the seam and interfere with the electrical component.
One type of EMI seam used in electronic packaging incorporates the use of gaskets or discrete springs. These devices are typically compressed between two surfaces of the cage, such as a cover and housing, making a continuous electrical bond from the housing through the gasket or spring to the cover. The springs and gaskets are typically made of flexible conductive materials, such as metal and/or modified elastomers. FIG. 1A shows a row of discrete spring fingers embedded in the wall of a Faraday cage. FIG. 1B shows the EMI seam between two walls of a Faraday cage with one wall comprising the row of discrete springs fingers of FIG. 1A. While spring fingers are used in many commercial applications, they may require customization, are susceptible to damage, and require many fasteners for retention.
Another type of EMI seam includes the fastening of a flat plate or cover to a flat frame or housing using the appropriate amount of fasteners necessary to make a continuous electrical bond at the perimeter. However, this EMI seam typically requires a large number of screws to prevent buckling of the flat plate to the flat frame. In military applications, screw spacing as close as one inch has been implemented. The flat plates must also be manufactured from relatively thick pieces of sheet metal that can withstand buckling. Both of these constraints result in increased hardware and manufacturing costs.
Various aspects of the present invention relate to methods and apparatus for providing inexpensive and practical EMI shielding for a variety of electrical components such as circuit card assemblies. In accordance with an exemplary embodiment of the present invention, the enclosure includes a frame member for housing the electrical components to be shielded. According to one embodiment, the frame member includes a front member, a back member, a bottom member and a top member wherein the sides of at least one of said members is tapered. The enclosure may further include covers that are attached to the sides of the front, back, bottom and top members to form a Faraday cage. The front and/or the back member may be configured to retain at least one electrical component, such as a circuit card assembly, within the Faraday cage. By securely attaching the covers to the tapered sides of the front, back, bottom and/or top members, a load is imparted to the covers thereby producing a tight and continuous EMI seam, effectively shielding the electrical components from EMI.
Other embodiments of the invention relate to a method of shielding an electrical component by compressively engaging covers to a frame member having at least one of two side faces that is tapered.